What should be done after pasting the complete configuration into the replacement device?

After pasting the complete configuration into the replacement device, it is essential to type 'copy run start' to save the configuration. This command copies the running configuration, which is what the device is currently using, into the startup configuration, which is the configuration that will be used when the device is rebooted.

Writing the configuration to the startup memory ensures that the changes are persistent across reboots. If this step is not taken, any configuration changes made may be lost if the device is restarted or powered down, leading to potential operational issues.

Other options such as resetting the device, disconnecting the power, or running a diagnostic check do not directly address the need to save the configuration to ensure that the device performs as expected following the configuration updates. Saving the configuration is a fundamental and necessary step to maintain the intended functionality of the network device.